720 Encyclopedia of the Solar System
of the first Kuiper Belt Object (KBO) was made in 1992
on the University of Hawaii 2.2-m telescope. Tremendous
advances have been made in detecting KBOs since then:
presently over 900 KBOs have been discovered. Using
several of the largest telescopes in the world, it was re-
cently found that the largest KBO known, 2003 UB 313 ,
has methane ice on its surface and a moon (Fig. 1). This
finding has challenged our definition of what is considered
to be a planet in our solar system. Another recent result
was the discovery of comets among the main-belt aster-
oids. The most recent of these, asteroid 118401 was dis-
covered by the 8-m Gemini-North telescope. Two other
comets in the main belt were detected previously by other
astronomers, and many more such comets are now thought
to exist in the asteroid main belt. If this is confirmed then
such comets were likely the main source of water deliv-
ered to the Earth during its formation. A final example
is the Near-Earth Object (NEO) designated 2004 MN 4 ,
which was discovered with the University of Arizona’s 2.3-
m telescope. For a short time at the end of December
2004, this NEO had the highest probability of any yet found
for colliding with Earth (Fig. 3). These discoveries demon-
strate the importance of ground-based astronomy, and they
will no doubt provide the scientific motivation for future
missions.
Solar system astronomers typically use telescopes built
for other fields of astronomy. However, during the 1970s,
NASA constructed ground-based telescopes to support its
planetary missions. NASA funded the construction of the
2.7-m McDonald telescope, the University of Hawaii 2.2-
m telescope, and the 3.0-m NASA Infrared Telescope
Facility (IRTF) to provide mission support, but currently
only the IRTF continues to be funded by NASA for that pur-
pose. NASA also provides funding for searches for NEOs
as part of a Congressional directive.
Telescopes are designed to collect and focus starlight
onto a detector. While conceptually simple, ground-based
observers have to contend with limitations imposed by
physics, the atmosphere, and technology. First, the col-
lecting area of a telescope is limited in size. The largest
optical telescope in the world presently has an equivalent
collecting area of an 11.8-m diameter mirror. Although
larger telescopes could be built, there are serious tech-
nical and financial difficulties to overcome. Larger tele-
scopes not only allow more light to be collected and put
onto the detector, they also allow sharper images to be
obtained at the diffraction limit of the telescope. Second,
the atmosphere limits observations to specific observing
“windows” where the atmosphere is transparent, and the
wavelength range 25μm to 350μm is largely inaccessi-
ble to ground-based observers because of water absorp-
tion bands. Third, for infrared observations, the thermal
emission of the atmosphere at wavelengths longer than 2.5
μm greatly reduces the sensitivity of observations. To over-
come the problems of atmospheric absorption and ther-
mal emission, it is necessary to go to high-mountain sites
such as Mauna Kea in Hawaii and Atacama in Chile, or to
use balloons, aircraft, or spacecraft. Fourth, atmospheric
seeing typically limits the sharpness of images to 0.25–
0.5 arcseconds at the best high-altitude sites. To achieveFIGURE 1 (a) Image of KBO UB 313 obtained with the 10-m
Keck II telescope with a laser guide star adaptive optics system.
With a diameter estimated to be about 2400 km, it is the largest
KBO known and is slightly larger than Pluto. It was recently
named Eris. This image shows that UB 313 has a satellite, as does
Pluto. (b) A near-infrared spectrum of UB 313 and Pluto. The
spectrum of Pluto was obtained with the 8-m Gemini North
telescope. Both objects have methane ice on their surface
(methane ice absorption marked with arrows), thus
strengthening the idea that there is a common origin for these
objects. (Courtesy of M. Brown and C. Trujillo.)